The present invention relates to photolithographic compounds and methods, and specifically to photosensitizers used in lithographic resists. Basically, photolithography comprises applying a thin coating of resist material, exposing the resist through a mask such that light energy strikes selected areas of the resist and then developing the resist. Development involves washing away either the exposed resist, where a positive tone resist is used, or washing away the unexposed portion of the resist where a negative tone resist is used. This technology has been used in the printing industry for many years, and has been used more recently in the manufacture of semiconductor devices.
Positive tone resists are gradually becoming the most important of the two type of resists because of the higher resolution and thermostability that positive resists offer. They also provide greater resistance to dry etching environments. Positive resists comprise a matrix resin into which is blended a photosensitizer. The matrix resin is usually a copolymer of phenol and formaldehyde, more specifically a mixture of cresols and formaldehyde, the generic term for this class of polymers being novolak. Novolak resins are soluble in common organic solvents and aqueous base solutions.
The photosensitizers used in positive tone resists are substituted diazo compounds. Suitable diazo compounds act as dissolution inhibitors for aqueous base development of the novolak resin. Films of novolak containing 15 to 20% by weight of a diazo-naphthoquinone sensitizer dissolve orders of magnitude more slowly in aqueous base solution than films of novolak alone. When the diazo compound is exposed to light on the other hand, it undergoes a series of reactions that culminate in the formation of an indene carboxylic acid. This photo product, unlike its precursor, is extremely soluble in aqueous base by virtue of the carboxylic acid functionality. Complete photo decomposition of the sensitizer results in a dissolution rate for the resin/sensitizer blend that is equal to or greater than the intrinsic dissolution rate of the novolak matrix resin alone. This photochemically generated difference in dissolution rate in aqueous base is exploited in the generation of relief images.
One class of diazo compound which have been widely used as lithographic photosensitizers are hydroxy compounds condensed with 1-oxo-2-diazo-naphthalene-sulfonic acid chlorides (diazo-naphthoquinone-sulfonyl chloride), with the sulfonic group typically occupying the 4 or 5 position. Generally speaking, the bigger the molecules of the hydroxy compounds condensed with the 1-oxo-2-diazo-naphthalene-sulfonic acid chloride are, the better suited are the resulting diazo-naphthoquinone-sulfonic acid esters. This is recognized in U.S. Pat. No. 3,046,121 to M. P. Schmidt, issued July 24, 1962. One specific hydroxy compound which has been very successfully used in diazo-naphthalene-sulfonic acid esters is trihydroxybenzophenone. Examples of diazo-naphthalene-sulfonic acid esters based on this compound are disclosed in U.S. Pat. No. 3,148,983 to Endermann et al. issued Sept. 15, 1964 and in U.S. Pat. No. 3,402,044 to Steinhoff et al. issued Sept. 17, 1968.
One drawback to the foregoing diazo ester is that it does not work well where the light used to expose the resist is in the mid UV range. The development of higher circuit density in microelectronic devices has led to a desire for improved resolution of optical projection through the use of higher energy, shorter wavelength radiation than that currently employed in the near UV spectral region (350 to 450 nm). A reduction in the exposure wavelength from near UV to the 313 mm mid UV emission level would produce a substantial increase in resolution.
Unfortunately, the ester of trihydroxybenzophenone and 1-oxo-2-diazo-naphthalene-sulfonic acid does not substantially bleach in the mid UV light range when it is exposed to light. In other words, the bleached sensitizer continues to absorb a significant amount of mid UV radiation. This substantially slows the bleaching process in that the upper molecular layers tend to "shade" the lower molecular layers. The resist is thus slower when mid UV light is used to expose the resist.